This USB cable adds a DB9 serial port to your computer or laptop. Works with MacOS X, Linux and Windows. Note that this provides (approximately) +-6V serial (RS232) not 5V serial (TTL), good for old devices like GPS, handhelds, programmers, etc with chunky DB9's on the side. Some cheaper RS232 cables 'cheat' and only give you 0-5V which wont work with some devices - this one has a true voltage doubler/inverter to make it more compatible.
These are tested and work great with bit-bang serial programmers and adapters - note that they are slow in comparison to hardware serial ports due to the high overhead of USB.
New! we just upgraded our adapters to be way more awesome. They're now FTDI-chip based for better performance and reliability. The drivers are rock solid and available for pretty much every operating system you will run across. Even better, inside are three visible LEDs: a power LED (letting you know its plugged in and working) and RX/TX LEDs that you can use to help debug your connection. These are now our favorite RS232 adapters!
Using our older "gray" PL2303 converter? It comes with a CD but we strongly suggest using the most recent drivers from the Prolific Website if you're running windows and the 'fixed' driver for Mac OSX or the version for MacOS 64 bit. If you're using Windows 7 and you have issues, try the Prolific driver version 1.013 (This is only for the older "PL2303" based converter)